1. Field of the Invention
The present invention relates to a mobile communication system, a mobile communication method, and a radio network controller which are configured so that a mobile station shifts from a first communication state to a second communication state. Specifically, the first communication state is a state in which the mobile station performs high speed downlink packet communication by using a high speed downlink packet communication compliant radio link, and performs uplink packet communication by using at least one radio link not including a high speed uplink packet communication compliant radio link. The second communication state is a state in which the mobile station performs high speed downlink packet communication by using a high speed downlink packet communication compliant radio link, and performs high speed uplink packet communication by using a high speed uplink packet communication compliant radio link.
2. Description of the Related Art
In a mobile communication system adopting an Enhanced Uplink (EUL) or a High Speed Uplink Packet Access (HSUPA) scheme, and a High Speed Downlink Packet Access (HSDPA) scheme defined in 3GPP, a mobile station UE is capable of performing high speed downlink packet communication (HSDPA communication) by using a HSDPA communication compliant radio link and is also performing high speed uplink packet communication (EUL communication) by using an EUL communication compliant radio link (see Published Japanese translation of a PCT application No. 2000-505261).
In the above-described mobile communication system, the mobile station UE cannot establish the EUL communication compliant radio link with a non EUL compliant base station which is not compliant with the EUL communication. Accordingly, when existing in a cell under the control of the non EUL compliant base station, the mobile station UE cannot perform EUL communication.
Specifically, when the mobile station UE is in a cell A under the control of a non EUL compliant base station #1 which is compliant with the HSDPA communication but not compliant with the EUL communication, as shown in FIG. 1, the mobile station UE is configured to perform HSDPA communication by using the HSDPA communication compliant radio link and uplink packet communication (R99 communication, usual uplink packet communication) by a Release 99 scheme by using a non EUL communication compliant radio link (a usual radio link) (first communication state). This is because the mobile station UE is incapable of performing EUL communication by using the EUL communication compliant radio link.
However, 3GPP does not define a trigger of shift of the mobile station UE from the first communication state to a second communication state in which the mobile station UE performs HSDPA communication by using the HSDPA communication compliant radio link, and EUL communication by using the EUL communication compliant radio link. For this reason, there is a problem that even when the mobile station UE which has been in the first communication state is currently present in a cell B under the control of an EUL compliant base station #2 compliant with both the HSDPA communication and the EUL communication, the mobile station UE is not capable of changing into the second communication state.